OK, I realized today that the Dashcam Icon was no longer visible. Tapping on my Sentry alerts did nothing. I tried to format the USB drive from the car menu, but all I got was the Dashcam Icon with a gray "X" on it, and then it would go away. I pulled the USB drive out, which now I remember, is just an adapter to the microSD.
I brought it back home and tried to plug it into my computer, but it didn't show at all. I used a different adapter (microSD to SD) and that didn't work either. Like the card wouldn't register as a plugged in drive.
I tried a handful of times, then I switched to the SD adapter again, and it showed up! I reformatted it, and now it's back in the car, and it's all working again.
Super strange. Maybe the microSD card is failling?